Fix web builds for itch

[NiklasEi]

The .zip files currently created by trunk are not compatible with itch.io, because they load resources from non-relative paths.

[idanarye]

Note for those who want to fix it manually:

1. Unzip the archive and edit index.html.
2. Find these two lines:

   <link rel="preload" href="/<game-name>-<some-hash>_bg.wasm" as="fetch" type="application/wasm" crossorigin="">
   <link rel="modulepreload" href="/<game-name>-<same-hash>.js"></head>

   And add a . before the / in the href.
3. Go to the end of the file, to this line:

   <script type="module">import init from '/<game-name>-<same-hash>.js';init('/<game-name>-<same-hash>.wasm');</script></body></html>

   And add a . in before the / in the string in the import statement and the starting passed to the init() function.
4. Upload the unzipped directory with the modified index.html (no need to re-zip it if you use butler)

[NiklasEi]

This is currently not possible with trunk (see trunk-rs/trunk#395). I will use a workaround that replaces the links in CI 🙈